High-frequency circuit and high-frequency package

ABSTRACT

A high-frequency circuit formed on a surface of a dielectric substrate includes: a signal strip formed on a first face of the dielectric substrate for transmitting a signal therethrough; a pair of ground strips formed on the first face astride the signal strip, with an interspace on each side of the signal strip; a ground conductor layer formed on a second face of the dielectric substrate, the second face being opposite to the first face; and a plurality of through-vias formed in the dielectric substrate astride the signal strip for electrically connecting the pair of ground strips to the ground conductor layer. First and second through-vias, which are a pair of opposing through-vias located closest to a terminating end of the signal strip, are disposed apart from each other by a distance smaller than a distance between any other pair of opposing through-vias.

[0001] This application is a continuation of International ApplicationPCT/JP03/15452, filed Dec. 3, 2003.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a high-frequency circuit whichis applicable to a high-frequency module utilizing a radiofrequency inthe microwave or millimeter range. More particularly, the presentinvention relates to a high-frequency circuit which effectively reducesradiation loss associated with a high-frequency signal.

[0004] 2. Description of the Background Art

[0005] A rapid increase in the number of users of wireless devices inrecent years underlines the need for an ability to utilize themillimeter range as a new frequency resource. There have been studiesconducted to realize millimeter-range distance measuring devices inanti-collision radar for automobiles or the like, in an attempt to takeadvantage of the short wavelength of the millimeter range. However,before practical applications of millimeter range devices can berealized, the ability to mass-produce low cost and compacthigh-frequency circuitry will generally be required.

[0006] In order to enable mass-production of low cost and compacthigh-frequency circuitry, various high-frequency packages have beenproposed. For example, a high-frequency package has been proposed inwhich a connection terminal is formed on a lower face of a package wherea signal strip line has been taken out by utilizing a throughholeconductor or the like which penetrates a dielectric substrate, such thatthe package can be surface-mounted, by solder reflow technique, onto thewiring provided on an external circuit substrate.

[0007]FIG. 9A is a schematic cross-sectional view illustrating aconventional high-frequency package having been surface-mounted to anexternal circuit substrate. FIG. 9B is a view illustrating a wiringpattern of conductors formed on an upper face of a dielectric substrate110. FIG. 9C is a view illustrating a wiring pattern of conductorsformed on a lower face of the dielectric substrate 101.

[0008] In FIG. 9A, the high-frequency package comprises a high-frequencyelement 110, a dielectric substrate 101, and a cover 109. Thehigh-frequency package is surface-mounted on an external circuitsubstrate 113. As shown in FIG. 9B, on the upper face of the dielectricsubstrate 101, a ground conductor layer 104, two signal strips 102 a,and a ground conductor region 104 b are formed. As shown in FIG. 9C, onthe lower face of the dielectric substrate 101, two signal strips 102 b,two ground strips 103 which are disposed so as to leave a predeterminedspace from the signal strips 102 b, and a ground conductor region 104 care formed. The signal strips 102 a, the ground conductor layer 104, andthe ground conductor region 104 c together constitute a groundedcoplanar waveguide structure. The signal strips 102 b, the ground strips103, and the ground conductor layer 104 together constitute anothergrounded coplanar waveguide structure. As used herein, a “strip” refersto a wiring piece of conductor.

[0009] One end of each signal strip 102 a is connected to thehigh-frequency element 110 via a wire 111. The wire 111 may be a ribbonor the like. The high-frequency element 111 may be mounted facedown, viaconductor bumps. In other words, the high-frequency element 110 may bemounted through wireless bonding, e.g., flip chip mounting. The otherend of each signal strip 102 a is connected to one end of acorresponding signal strip 102 b, by means of a through-via forconnection purposes (hereinafter simply referred to as a “through-via”)112 which penetrates the dielectric substrate 101. Thus, ahigh-frequency signal which is output from the high-frequency element110 or a high-frequency signal which is input to the high-frequencyelement 110 is transmitted via the wires 111, the signal strips 102 a,the through-vias 112, and the signal strips 102 b, without beinggrounded. Note that “through-via” is synonymous to “through conductor”for the purpose of the present specification.

[0010] On the upper face of the dielectric substrate 101, the groundconductor region 104 b is disposed directly under the high-frequencyelement 110, so as to be electrically connected to the ground conductorlayer 104. Via a plurality of through-vias 104 d penetrating thedielectric substrate 101, the ground conductor region 104 b is connectedto the ground conductor region 104 c formed on the lower face of thedielectric substrate 101. The ground conductor region 104 c iselectrically connected to the ground strip 103. Thus, a high-frequencyground is provided in the ground conductor region 104 d. An arbitrarynumber of through-vias 116 z are formed between the ground conductorlayer 104 and the respective ground strips 103. The through-vias 116 zelectrically connect the ground strips 103 to the ground conductor layer104, whereby a better high-frequency grounding ability is provided.

[0011]FIG. 10A is a view illustrating an exemplary wiring pattern ofconductors formed on an upper face of the external circuit substrate113. FIG. 10B is a view illustrating an exemplary wiring pattern ofconductors formed on a lower face of the external circuit substrate 113.

[0012] The external circuit substrate 113 is a substrate on which thehigh-frequency package is to be surface-mounted. As shown in FIG. 10A,on the upper face of the external circuit substrate 113, two signalstrips 114, two ground strips 116, and a ground conductor region 116 bare formed. Interspaces are provided between each signal strip 114 andthe ground strips 116. As shown in FIG. 10B, on the lower face of theexternal circuit substrate 113, a ground conductor layer 115 is formed.

[0013] Each signal strip 114 is electrically connected to acorresponding signal strip 102 b via solder 117. Each ground strip 116is electrically connected to a corresponding ground strip 103 via solder117.

[0014] The ground conductor region 116 b is disposed so as to comedirectly below the high-frequency element 110. The ground conductorregion 116 b is electrically connected to the ground conductor region104 c via solder 117. The ground conductor region 116 b is connected tothe ground conductor layer 115 by means of through-vias 116 dpenetrating the external circuit substrate 113. As a result, ahigh-frequency ground is provided in the ground conductor region 116 b.An arbitrary number of through-vias 116 are formed between the groundconductor layer 115 and the respective ground strips 116. Thethrough-vias 116 electrically connect the ground strips 116 to theground conductor layer 115, whereby a better high-frequency groundingability is provided.

[0015] Due to the aforementioned strip line structure, the externalcircuit substrate 113 functions as a grounded coplanar waveguide inwhich a high-frequency signal which is output from the high-frequencyelement 110 or a high-frequency signal which is input to thehigh-frequency element 110 can be transmitted without being grounded.Note that the ground conductor layer 115 may be provided inside theexternal circuit substrate 113. Further note that the external circuitsubstrate 113 will function as a microstrip line if the ground strips116 are not provided.

[0016] Based on the above-described structure, which allows thehigh-frequency element 110 to be mechanically and electrically connectedto the dielectric substrate 101, a compact high-frequency package isprovided. Since signal strips are taken out on the lower face of thehigh-frequency package, it is easy to surface-mount the high-frequencypackage on the external circuit substrate. Thus, by employing theabove-described high-frequency package, it is possible to provide lowcost and compact high-frequency circuitry, with good mass producibility.

[0017] However, when the high-frequency package having the abovestructure is employed for transmitting a high-frequency signal, e.g., asignal in the millimeter range, losses may occur in various places.Therefore, the high-frequency package must be designed so as to minimizetransmission loss of the high-frequency signal.

[0018]FIG. 11 is a cross-sectional view of the dielectric substrate 101shown in FIGS. 9B and 9C, taken at line A-B. As shown in FIG. 11, thethrough-vias 116 z are arranged in opposing rows astride the signalstrips 102 b. The ground strips 103, the ground conductor layer 104, thethrough-vias 116 z, and the signal strips 102 b together constitute atransmission line such as a grounded coplanar waveguide.

[0019] In a transmission line such as the grounded coplanar waveguideshown in FIG. 11, a waveguide surrounding the signal strips 102 b iscreated by the ground strips 103, the ground conductor layer 104, andthe through-vias 116 z connecting therebetween. When such a waveguide iscreated, the high-frequency package must be designed so thattransmission does not occur in a waveguide mode (i.e., a mode oftransmission via the waveguide) at a frequency of the high-frequencysignal to be transmitted. Otherwise, at each frequency of thehigh-frequency signal to be transmitted, the fundamental transmissionmode (hereinafter simply referred to as the “transmission mode”) will beconverted to the waveguide mode, thereby resulting in transmissionlosses.

[0020] It is known that the waveguide mode can be suppressed byprescribing a distance W between a pair of opposing through-vias 116 zto be half of an effective wavelength that corresponds to a designedfrequency within the dielectric substrate 101. Assuming that thedielectric substrate 101 has a dielectric constant ∈, the effectivewavelength of electromagnetic waves in the dielectric substrate 101 canbe calculated by dividing a wavelength of the electromagnetic waves in afree space by ∈^(1/2).

[0021] M, ITO et al., “Analysis of Millimeter-wave Packaging StructureUsing Electromagnetic Simulator and Its Application to W-band Package”,technical report of The Institute of Electronics, Information andCommunication Engineers, ED2000-154 MW2000-107 (2000-09), pp.55-60(hereinafter referred to as “Publication 1”) shows an example designwhich takes waveguide mode suppression into consideration. In theexample design described in Publication 1, a dielectric substrate havinga dielectric constant of 7.5 is used, such that the distance between apair of through-vias across a signal strip is 0.5 mm at the minimum. Asillustrated in FIG. 6 of Publication 1, in the case where the designexample of Publication 1 is used, deteriorations in the transmissioncharacteristics occur in the neighborhood of 100 GHz to 120 GHz. Theminimum opposing distance of 0.5 mm between a pair of through-vias isequivalent to half of the effective wavelength at a frequency of about102 GHz. Publication 1 attributes such characteristics deteriorations toan increased loss due to a parasitic waveguide mode. In contrast, it canbe seen that the transmission characteristics do not deteriorate in afrequency range up to about 90 GHz by using the opposing distanceexemplified in Publication 1. Thus, from the description of Publication1, it can be seen that the waveguide mode can be suppressed if theopposing distance W between each pair of through-vias is prescribed tobe half of the effective wavelength corresponding to the designedfrequency.

[0022] Moreover, radiation losses occurring at connections betweensignal strips and an external circuit substrate are also problems. Atsuch connections, a high-frequency signal which has been transmitted inthe fundamental mode inclines toward a parallel plane mode (which is ahigher-order mode) due to an overlap between the ground conductor layerof the connection terminal and the ground conductor layer of theexternal circuit substrate, thus causing radiation loss.

[0023] Japanese Patent No. 3046287 (hereinafter referred to as“Publication 2”) describes an exemplary method for reducing theaforementioned radiation loss. Specifically, Publication 2 proposesremoving a portion of the conductor opposing the signal strips from partof the ground conductor layer of the connection terminal, to reduceradiation loss. Based on such a structure, the overlap between theground conductor layer of the connection terminal and the groundconductor layer of the external circuit substrate is reduced, wherebythe parallel plane mode is suppressed. As a result, a high-frequencypackage which can reduce radiation losses at connections can berealized.

[0024] Publication 1 also discloses an exemplary method of reducingradiation losses at connections. Publication 1 includes a detaileddiscussion of a parallel plane mode which is induced by an overlapbetween the ground conductor layer of the connection terminal and theground conductor layer of the external circuit substrate. In Publication1, at a connection boundary of the ground conductor layer of theconnection terminal that lies closest to the substrate, a semicolumnarshaped connection conductor that penetrates through the end face isformed so that proper short-circuiting will occur all the way up to theultrahigh-frequency band. By forming such a connection conductor thatpenetrates through the end face, the parallel plane mode is suppressed,whereby radiation loss can be reduced.

[0025] However, the above-described conventional techniques cannotattain complete elimination of transmission losses, and would presentfurther problems in practice.

[0026] For example, the high-frequency package disclosed in Publication2 has the problem of an increased module size. Downsizing is anessential requirement in any present-day high-frequency device; however,the high-frequency package disclosed in Publication 2 fails to satisfythis need. In the high-frequency package disclosed in Publication 2, aportion of the ground conductor layer formed closest to an end face ofthe dielectric substrate is removed. Such a partial removal of theground conductor layer means that, in view of possible influences on thehigh-frequency transmission characteristics, it is undesirable toprovide a cover, composed of a metal, ceramic, resin, or like materials,above the removed portion. For example, consider a case where a resinsubstrate is used as a dielectric substrate, under wiring process rulessuch that wiring can only be provided in portions at least 100micrometers away from ends of the substrate and that the through-viashave a land diameter of 600 micrometers. Removing portions of the groundconductor layer between through-vias in this case would mean removing anarea which is at least 700 micrometers long or more for each through-viaexisting at an end of the substrate. In this case, since one isprohibited from providing a cover over each of such areas which are 700micrometers long or more, the size of the high-frequency package willhave to be increased, given that the size of the MMIC to be mounted isno more than about one square millimeter. Therefore, the methoddescribed in Publication 2 cannot be adopted in whole.

[0027] The high-frequency package disclosed in Publication 1 hasproblems in terms of reliability and reproducibility because, whenproducing a high-frequency circuit by using a resin substrate or a hightemperature-sintered ceramic substrate, etc., it would detract fromreliability and reproducibility to form the through-vias in such amanner that the interior is exposed at the ends of the substrate.

SUMMARY OF THE INVENTION

[0028] Therefore, an object of the present invention is to provide ahigh-frequency circuit constructed on a dielectric substrate defining ahigh-frequency transmission line composed of a signal strip and groundstrips, the high-frequency circuit having a wiring pattern such that,when the wiring substrate is connected to an external circuit substrate,the transmission loss of a high-frequency signal occurring at theconnection can be reduced. The high-frequency circuit is shaped so as torequire a minimum volumetric space, and can be provided withoutemploying any special process.

[0029] In order to attain the object mentioned above, the inventor hasfound that the aforementioned transmission loss can be effectivelyreduced by decreasing the distance between a pair of opposingthrough-vias located closest to an end of the high-frequencytransmission line, thereby accomplishing the present invention.

[0030] A first aspect of the present invention is directed to ahigh-frequency circuit formed on a surface of a dielectric substrate,comprising: a signal strip formed on a first face of the dielectricsubstrate for transmitting a signal therethrough; a pair of groundstrips formed on the first face astride the signal strip, with aninterspace on each side of the signal strip; a ground conductor layerformed on a second face of the dielectric substrate, the second facebeing opposite to the first face; and a plurality of through-vias formedin the dielectric substrate astride the signal strip for electricallyconnecting the pair of ground strips to the ground conductor layer,wherein, among the plurality of through-vias, first and secondthrough-vias which are a pair of opposing through-vias located closestto a terminating end of the signal strip are disposed apart from eachother by a distance smaller than a distance between any other pair ofopposing through-vias.

[0031] In conventional techniques, the waveguide mode is suppressed in aperiodic manner by disposing pairs of opposing through-vias with aconstant period, the distance between each pair of opposing through-viasbeing half of the effective wavelength corresponding to the designedfrequency. However, in those conventional techniques, periodicity isbroken at the terminating end of the grounded coplanar waveguidestructure, such that the waveguide mode suppressing effect provided bythe opposing through-vias is weakened, and the transmission loss due tothe waveguide mode is increased. Specifically, in a region lying closerto the substrate end than an imaginary line connecting the centers ofthe pair of through-vias provided at the terminating end, the distancebetween the through-vias defining side walls of the waveguide graduallyincreases, thus gradually lowering the cutoff frequency for thewaveguide mode to and resulting in an increased transmission loss.

[0032] According to the first aspect of the present invention, thedistance between a pair of opposing through-vias at the terminating endof the grounded coplanar waveguide is less than a distance required forsuppressing the waveguide mode at the transmission frequency. Therefore,the cutoff frequency can be prevented from lowering even in the regionlying closer to the substrate end than an imaginary line connecting thecenters of the pair of through-vias provided at the terminating end.Thus, a high-frequency circuit which can minimize an increase in thetransmission loss can be provided. The high-frequency circuit accordingto the present invention also has an advantage in that the circuit scaleis not increased as compared to the circuit scale used in theconventional techniques.

[0033] Moreover, according to the first aspect of the present invention,the distance between the pair of opposing through-vias provided at theterminating end is reduced, whereby the grounding ability of the groundconductor layer formed on the second face of the dielectric substrate isimproved. Therefore, it becomes easier to maintain groundingcharacteristics such that proper short-circuiting will occur all the wayup to the ultrahigh-frequency band even at the connection boundary(lying closest to the substrate end) of a ground conductor layer on anexternal circuit substrate. As a result, the parallel plane mode issuppressed, whereby the radiation loss can be reduced. Thehigh-frequency circuit according to the present invention also has anadvantage in that it can be produced without employing any specialprocess beyond conventional techniques, and can be produced by employinga commonly-used set of process rules.

[0034] For example, the distance between the first and secondthrough-vias may be less than ½ of an effective wavelength correspondingto a designed frequency, and the distance between any other pair ofopposing through-vias may be equal to or less than ½ of the effectivewavelength corresponding to the designed frequency.

[0035] Preferably, the terminating end of the signal strip is near anend of the dielectric substrate, and the first and second through-viasare disposed so that a distance between portions thereof that lieclosest to the end of the dielectric substrate is less than ½ of theeffective wavelength corresponding to the designed frequency.

[0036] Thus, the pair of through-vias at the terminating end aredisposed so that a distance between portions thereof that lie closest toan end of the dielectric substrate is less than ½ of the effectivewavelength corresponding to the designed frequency. As a result, thewaveguide mode can be suppressed more effectively.

[0037] Preferably, the first and second through-vias are each disposedso as to be away from an end of the ground conductor layer by a distancewhich is less than ¼ of the effective wavelength corresponding to thedesigned frequency.

[0038] Thus, the radiation loss can be further reduced. Conversely, ifthe first and second through-vias provided at the terminating end of thesignal strip were placed in a region away from the terminating end ofthe ground conductor layer by a distance which is equal to or greaterthan ¼ of the effective wavelength corresponding to the designedfrequency, a higher-mode resonance would occur at a frequency whose ¼wavelength is equal to the effective distance from the pair ofthrough-vias closest to the substrate end to the end of the groundconductor layer, thus increasing the radiation loss in a frequency rangenear the resonance frequency. However, according to the above-describedembodiment of the present invention, the first and second through-viasare each disposed so as to be away from the end of the ground conductorlayer by a distance which is less than ¼ of the effective wavelength. Asa result, the higher-mode resonance can be prevented, thereby reducingthe radiation loss in a frequency range near the resonance frequency.The present invention is advantageous over the conventional techniquesnot only because the present invention provides a solution based on acircuit structure which requires a less volumetric space, but alsobecause no special processes are required during manufacture accordingto the present invention.

[0039] Preferably, the signal strip is narrower at the terminating endthan at any other portion.

[0040] Thus, signal transmission with subdued reflections is enabled.Traditionally, in order to surface-mount a high-frequency package, itwould be necessary to connect the signal strip on the dielectricsubstrate of the high-frequency package to a signal strip on an externalcircuit substrate, and connect the ground strip on the dielectricsubstrate to a ground strip on the external circuit substrate, i.e., thetwo coplanar waveguides must be interconnected. However, as a result ofthis, the capacitance existing between signal strip and the groundconductor layer opposing the signal strip will exert a substantialinfluence over the device characteristics, because the transmission modeof the high-frequency transmission line behaves in a microstrip-linefashion everywhere but the terminating end of the high-frequencytransmission line. Therefore, according to the above-describedembodiment of the present invention, the width of the signal strip isnarrowed at the terminating end of the high-frequency transmission line,so that the capacitance existing between signal strip and the groundconductor layer is decreased, and conversely, the capacitance existingbetween the signal strip and the ground strips on both sides of thesignal strip is increased. As a result, a smooth transmission modetransition can occur, thus enabling signal transmission with subduedreflections. The present invention is advantageous over the conventionaltechniques not only because the present invention provides a solutionbased on a circuit structure which requires a less volumetric space, butalso because no special processes are required during manufactureaccording to the present invention.

[0041] Preferably, an interspace between the signal strip and eachground strip is narrower at the terminating end of the signal strip thanat any other portion.

[0042] Thus, the capacitance existing between the signal strip and theground strips on both sides of the signal strip is increased, so that asmooth transmission mode transition can occur, thus enabling signaltransmission with subdued reflections. The present invention isadvantageous over the conventional techniques not only because thepresent invention provides a solution based on a circuit structure whichrequires a less volumetric space, but also because no special processesare required during manufacture according to the present invention.

[0043] Preferably, the dielectric substrate is a resin substrate havinga low dielectric constant.

[0044] Thus, the effect according to the present invention can bemaximized. It is common practice to use a resin substrate or a ceramicsubstrate as the dielectric substrate. However, the higher dielectricconstant the material composing the dielectric substrate has, theshorter the effective wavelength within the dielectric substratebecomes. Employing a dielectric substrate having a high dielectricconstant would require a high precision in the wiring pattern formation,possibly causing characteristic fluctuations due to manufacturalfluctuations. Therefore, according to the above-described embodiment ofthe present invention, a dielectric substrate having a low dielectricconstant is used, thereby reducing the characteristic fluctuations andmaking it possible to obtain the expected effects. The present inventionis advantageous over the conventional techniques not only because thepresent invention provides a solution based on a circuit structure whichrequires a less volumetric space, but also because no special processesare required during manufacture according to the present invention.

[0045] Preferably, portions of the ground conductor layer interposedbetween the first and the second through-via and opposing the signalstrip are eliminated. Alternatively, within a region extending nearer toan end of the substrate than to a region interposed between the firstand second through-vias, portions of the ground conductor layer opposingthe signal strip are eliminated. Alternatively, portions of the groundconductor layer opposing the signal strip are removed.

[0046] Thus, the parallel plane mode is suppressed, and the radiationloss can be reduced. The present invention requires no special processesduring manufacture over conventional techniques. A particularly usefulembodiment is the embodiment in which, within a region extending nearerto an end of the substrate than to a region interposed between the firstand second through-vias, portions of the ground conductor layer opposingthe signal strip are removed. The reason is that this makes it possibleto place a cover over the first and second through-vias, which in itselfis advantageous in terms of downsizing the high-frequency package.

[0047] A second aspect of the present invention is directed to ahigh-frequency package into which an integrated circuit is packaged,comprising: a high-frequency element composed of the integrated circuitfor processing a high-frequency signal; and a dielectric substrate onwhich the high-frequency element is mounted, wherein the dielectricsubstrate includes: a signal strip formed on a first face of thedielectric substrate for transmitting a signal therethrough; a pair ofground strips formed on the first face astride the signal strip, with aninterspace on each side of the signal strip; a ground conductor layerformed on a second face of the dielectric substrate, the second facebeing opposite to the first face; and a plurality of through-vias formedin the dielectric substrate astride the signal strip for electricallyconnecting the pair of ground strips to the ground conductor layer,wherein, among the plurality of through-vias, first and secondthrough-vias which are a pair of opposing through-vias located closestto a terminating end of the signal strip are disposed apart from eachother by a distance smaller than a distance between any other pair ofopposing through-vias.

[0048] For example, the distance between the first and secondthrough-vias may be less than ½ of an effective wavelength correspondingto a designed frequency, and the distance between any other pair ofopposing through-vias may be equal to or less than ½ of the effectivewavelength corresponding to the designed frequency.

[0049] Preferably, the terminating end of the signal strip is near anend of the dielectric substrate, and the first and second through-viasare disposed so that a distance between portions thereof that lieclosest to the end of the dielectric substrate is less than ½ of theeffective wavelength corresponding to the designed frequency.

[0050] The high-frequency package may further comprise a mounting-sidedielectric substrate on which the dielectric substrate is mounted,wherein the mounting-side dielectric substrate includes: a mounting-sidesignal strip formed on a first mounting face of the mounting-sidedielectric substrate so as to be connected to the signal strip fortransmitting the signal therethrough; a pair of mounting-side groundstrips formed on the first mounting face astride the mounting-sidesignal strip, with an interspace on each side of the mounting-sidesignal strip; a mounting-side ground conductor layer formed on a secondmounting face of the mounting-side dielectric substrate, the secondmounting face being opposite to the first mounting face; and a pluralityof mounting-side through-vias formed in the mounting-side dielectricsubstrate astride the mounting-side signal strip for electricallyconnecting the pair of mounting-side ground strips to the mounting-sideground conductor layer, wherein, among the plurality of mounting-sidethrough-vias, first and second mounting-side through-vias which are apair of opposing mounting-side through-vias located closest to aterminating end of the mounting-side signal strip are disposed apartfrom each other by a distance smaller than a distance between any otherpair of opposing mounting-side through-vias.

[0051] For example, the distance between the first and secondmounting-side through-vias may be less than ½ of an effective wavelengthcorresponding to a designed frequency, and the distance between anyother pair of opposing mounting-side through-vias may be equal to orless than ½ of the effective wavelength corresponding to the designedfrequency.

[0052] Preferably, the terminating end of the mounting-side signal stripis near an end of the mounting-side dielectric substrate, and the firstand second mounting-side through-vias are disposed so that a distancebetween portions thereof that lie closest to the end of themounting-side dielectric substrate is less than ½ of the effectivewavelength corresponding to the designed frequency.

[0053] Preferably, the high-frequency package further comprises a coverfor protecting the high-frequency element.

[0054] A third aspect of the present invention is directed to ahigh-frequency circuit formed on a surface of a dielectric substrate onwhich a high-frequency package is to be surface-mounted, thehigh-frequency package having a coplanar waveguide formed on a lowerface thereof, comprising: a signal strip formed on a first face of thedielectric substrate so as to be connected to the high-frequency packagefor transmitting a signal therethrough; a pair of ground strips formedon the first face astride the signal strip, with an interspace on eachside of the signal strip; a ground conductor layer formed on a secondface of the dielectric substrate, the second face being opposite to thefirst face; and a plurality of through-vias formed in the dielectricsubstrate astride the signal strip for electrically connecting the pairof ground strips to the ground conductor layer, wherein, among theplurality of through-vias, first and second through-vias which are apair of opposing through-vias located closest to a terminating end ofthe signal strip are disposed apart from each other by a distancesmaller than a distance between any other pair of opposing through-vias.

[0055] For example, the distance between the first and secondthrough-vias may be less than ½ of an effective wavelength correspondingto a designed frequency, and the distance between any other pair ofopposing through-vias may be equal to or less than ½ of the effectivewavelength corresponding to the designed frequency.

[0056] Preferably, the terminating end of the signal strip is near anend of the dielectric substrate, and the first and second through-viasare disposed so that a distance between portions thereof that lieclosest to the end of the dielectric substrate is less than ½ of theeffective wavelength corresponding to the designed frequency.

[0057] According to the second and third aspects of the presentinvention, the waveguide mode and the parallel plane mode during ahigh-frequency signal transmission can be suppressed, whereby theradiation loss can be reduced, thus providing a practical advantage. Thepresent invention not only provides a solution based on a circuitstructure which requires a less volumetric space, but also requires nospecial processes during manufacture.

[0058] In the high-frequency circuit according to the present invention,the through-via may be shaped as a circular column, a rectangularcolumn, a triangular column, or a hexagonal column. For example, in thecase where the through-via is a rectangular column, a triangular column,or a hexagonal column, an excellent reduction in radiation loss can beobtained by disposing the first and second through-vias so that portionsthereof that lie closest to an end of the dielectric substrate are apartby a distance which is shorter than that between any other pair ofopposing through-vias.

[0059] These and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0060]FIG. 1A is a schematic cross-sectional view illustrating thestructure of a high-frequency circuit according to a first embodiment ofthe present invention;

[0061]FIG. 1B is a plan view showing a wiring pattern on a lower face ofthe high-frequency circuit shown in FIG. 1A;

[0062]FIG. 2A is an enlarged view showing a wiring pattern on a lowerface of a dielectric substrate;

[0063]FIG. 2B is a cross-sectional view illustrating the dielectricsubstrate 1 shown in FIG. 2A, taken at line a-b;

[0064]FIG. 2C is a cross-sectional view illustrating the dielectricsubstrate 1 shown in FIG. 2A, taken at line c-d;

[0065]FIG. 3 is an enlarged view showing a wiring pattern on ahigh-frequency circuit according to a second embodiment of the presentinvention;

[0066]FIG. 4 is an enlarged view showing a wiring pattern on ahigh-frequency circuit according to a third embodiment of the presentinvention;

[0067]FIG. 5A is an enlarged view showing a wiring pattern on a lowerface of a dielectric substrate 1 according to a fourth embodiment of thepresent invention;

[0068]FIG. 5B is an enlarged view showing a wiring pattern on an upperface of the dielectric substrate 1 shown in FIG. 5A;

[0069]FIG. 6A is a schematic cross-sectional view illustrating ahigh-frequency package according to a fifth embodiment of the presentinvention having been surface-mounted on an external circuit substrate;

[0070]FIG. 6B is a view illustrating a wiring pattern of conductorsformed on an upper face of a dielectric substrate 1 shown in FIG. 6A;

[0071]FIG. 6C is a view illustrating a wiring pattern of conductorsformed on a lower face of the dielectric substrate 1 shown in FIG. 6A;

[0072]FIG. 7A is a view illustrating a wiring pattern of conductorsformed on an upper face of an external circuit substrate 13 shown inFIG. 6A;

[0073]FIG. 7B is a view illustrating a wiring pattern of conductorsformed on a lower face of the external circuit substrate 13 shown inFIG. 6A;

[0074]FIG. 8 is a view showing a generic wiring pattern on a wiringsubstrate on which a high-frequency element was mounted for evaluationsas discussed in Examples;

[0075]FIG. 9A is a schematic cross-sectional view illustrating aconventional high-frequency package having been surface-mounted on anexternal circuit substrate;

[0076]FIG. 9B is a view illustrating a wiring pattern of conductorsformed on an upper face of a dielectric substrate 101 shown in FIG. 9A;

[0077]FIG. 9C is a view illustrating a wiring pattern of conductorsformed on a lower face of the dielectric substrate 101 shown in FIG. 9A;

[0078]FIG. 10A is a view illustrating an exemplary wiring pattern ofconductors formed on an upper face of an external circuit substrate 113;

[0079]FIG. 10B is a view illustrating an exemplary wiring pattern ofconductors formed on a lower face of the external circuit substrate 113;and

[0080]FIG. 11 is a cross-sectional view of the dielectric substrate 101shown in FIGS. 9B and 9C, taken at line A-B.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0081] (First Embodiment)

[0082]FIG. 1A is a schematic cross-sectional view illustrating thestructure of a high-frequency circuit according to a first embodiment ofthe present invention. FIG. 1B is a plan view showing a wiring patternon a lower face of the high-frequency circuit shown in FIG. 1A.

[0083] In FIGS. 1A and 1B, the high-frequency circuit according to thefirst embodiment includes a dielectric substrate 1, a signal strip 2,two ground strips 3, aground conductor layer 4, a plurality ofthrough-vias 5 a, and two through-vias 5 b. The signal strip 2 and theground strips 3 are formed on a lower face (which may conveniently bereferred to as the “first face”) of the dielectric substrate 1. Theground conductor layer 4 is formed on an upper face (which mayconveniently be referred to as the “second face”) of the dielectricsubstrate 1.

[0084] The signal strip 2 is formed in a middle portion of the lowerface of the dielectric substrate 1. The two ground strips 3 are formedon the lower face of the dielectric substrate 1 so as to be parallel toeach other, with an arbitrary interspace being left between the signalstrip 2 and the ground strips 3. The ground conductor layer 4 is formedon the upper face of the dielectric substrate 3 so as to extend parallelto the signal strip 2 and the ground strips 3. Based on anelectromagnetic field distribution between the signal strip 2 and eachground conductor, the transmission characteristics of the groundedcoplanar waveguide structure are determined.

[0085] The through-vias 5 a and 5 b are formed so as to extend from theupper face, through to the lower face of the dielectric substrate 1.Each of the through-vias 5 a and 5 b may be composed of, for example, aplating or the like provided on an inner wall which is created bydrilling a hole in the dielectric substrate 1. The through-vias 5 a and5 b electrically connect the ground strips 3 to the ground conductorlayer 4.

[0086] An opposing distance W between through-vias 5 a straddling thesignal strip 2 is equal to or less than ½ of an effective wavelengthcorresponding to a designed frequency within the dielectric substrate 1.As used herein, the “designed frequency” is defined as an upper limitfrequency value of the frequency range of a high-frequency signal to betransmitted through the signal strip 2. For example, in the case wherean active device such as a high-frequency element is connected to thesignal strip 2, a high-frequency signal will be input to or output fromthe active device by being transmitted through the signal strip 2; inthis case, the upper limit frequency value of the frequency range of thehigh-frequency signal to be input to or output from the active device isthe “designed frequency”. An effective wavelength of the electromagneticwaves in the dielectric substrate 101, assuming that the dielectricsubstrate 101 has a dielectric constant ∈, can be calculated by dividinga wavelength of the electromagnetic waves in a free space by ∈^(1/2). Byprescribing the opposing distance W between through-vias 5 a to be equalto or less than ½ of the effective wavelength corresponding to thedesigned frequency within the dielectric substrate 1, the waveguide modecan be suppressed. For example, in the case where a liquid crystalpolymer having a dielectric constant of 3 is used for the dielectricsubstrate 1, if the minimum distance between opposing through-vias 5 ais 1000 micrometers, the cutoff frequency for the waveguide mode in thetransmission line structure is about 85 GHz. Therefore, in order totransmit a signal in a frequency range of 85 GHz or less, the waveguidemode can be suppressed and hence the transmission loss can be reduced byprescribing the opposing distance W between through-vias 5 a to be 1000micrometers.

[0087] An essential point in the present invention is that an opposingdistance Wa between the pair of through-vias 5 b that are located theclosest to a terminating end (shown as the leftmost end in FIG. 1B) ofthe signal strip 2 is prescribed to be shorter than a distance which isrequired to ensure substantial elimination of the waveguide mode in thetransmission line structure. Note that the opposing distance Wa is to bemeasured between the inner edges of the through-vias 5 b, across thesignal strip 2.

[0088] Hereinafter, referring to FIGS. 2A, 2B, and 2C, the role whichthe pair of through-vias (“first and second through-vias”) 5 b,straddling the signal strip 2 and being formed close to the terminatingend of the signal strip 2, plays in the high-frequency circuit accordingto the present invention will be specifically described. FIG. 2A is anenlarged view showing a wiring pattern on a lower face of the dielectricsubstrate 1. FIG. 2B is a cross-sectional view illustrating thedielectric substrate 1 shown in FIG. 2A, taken at line a-b. FIG. 2C is across-sectional view illustrating the dielectric substrate 1 shown inFIG. 2A, taken at line c-d.

[0089] In FIG. 2A, the high-frequency circuit has a high-frequencytransmission line structure which can be divided into three regions A,B, and C, which are defined as follows. Region A is an area whichextends from an imaginary line connecting the centers 7 of the pair ofthrough-vias 5 b toward the center of the substrate. Region B is an areawhich extends between the imaginary line connecting the centers 7 of thepair of through-vias 5 b and an imaginary line connecting “far ends” ofthe through-vias 5 b, i.e., portions that lie closest to an end of thesubstrate (away from the center of the substrate). Region C is an areain which the signal strip 2 is no longer interposed between thethrough-vias 5 b.

[0090] In region A, an arbitrary number of through-vias 5 a areperiodically formed away from the through-vias 5 b, toward the center ofthe substrate (see FIG. 1B). Therefore, region A can be considered as anideal transmission line for suppressing the waveguide mode in a periodicmanner with respect to the direction of transmission. Then, in terms ofthe entire transmission line, the cutoff frequency for the waveguidemode will depend upon the minimum opposing distance Wa between the pairof through-vias 5 b that functions to suppress the waveguide mode at aposition closest to an end of the substrate.

[0091] In region B, if the through-vias 5 b are of a commonly-usedcolumnar shape, the opposing distance Wb between the pair ofthrough-vias 5 b gradually increases toward the end of the substrate,until finally reaching Wc. The cutoff frequency for the waveguide modedecreases as the opposing distance Wb increases. Therefore, in order toreduce the radiation loss over the entire transmission line, it isnecessary to minimize the radiation loss in region B by suppressing thewaveguide mode in region B.

[0092] According to the present invention, in order to suppress thewaveguide mode in region B, the opposing distance Wa between the pair ofthrough-vias 5 b is prescribed to be shorter than the opposing distanceW between any pair of through-vias 5 a which is required for eliminatingthe waveguide mode in region A.

[0093] Specifically, in the first embodiment, the opposing distance Wabetween the pair of through-vias 5 b is prescribed to be less than ½ ofthe effective wavelength corresponding to the designed frequency. As aresult, the waveguide mode is suppressed even in region B, therebyreducing the radiation loss over the entire transmission line.

[0094] Due to the relatively short opposing distance between the pair ofthrough-vias 5 b formed at a terminating end of the signal strip, thegrounding ability of the ground conductor layer 4 (which is formed onupper face on the dielectric substrate 1) is improved. Therefore, propershort-circuiting will occur all the way up to the ultrahigh-frequencyband at a connection boundary between the dielectric substrate 1 and theexternal circuit substrate. This makes it possible to suppress theparallel plane mode induced by an overlap between the ground conductorlayer 4 and a ground conductor layer (see the ground conductor layer 15in FIG. 6A, described later) which is formed on a side of the externalcircuit substrate opposing the signal strip 2 on the lower face of thedielectric substrate 1, whereby the radiation loss can be reduced.

[0095] Since it is unnecessary to form the ground conductor layer 4 onthe upper face of the dielectric substrate 1 into any special shape,there is no need for an extra circuit area. As a result, it is possibleto maintain a circuit structure which requires a minimum volumetricspace while reducing the radiation loss. Since no pair of through-vias 5b is partially exposed at an end face of the dielectric substrate 1, itis possible to reduce the radiation loss without employing any specialprocess, thus providing a practical advantage.

[0096] In the above embodiment, the distance Wa between the pair ofopposing through-vias 5 b is prescribed to be less than ½ of theeffective wavelength corresponding to the designed frequency. Morepreferably, the through-vias 5 b are disposed so that the opposingdistance Wc between portions thereof that lie closest to an end of thesubstrate (i.e., the points defining an imaginary border line betweenregions B and C) is less than ½ of the effective wavelengthcorresponding to the designed frequency. As a result, suppression of thewaveguide mode can be attained anywhere in region B, so that a moreeffective reduction in radiation loss can be attained. In order toprescribe the distance Wc between portions of the opposing through-vias5 b that lie closest to an end of the substrate to be less than ½ of theeffective wavelength corresponding to the designed frequency, the pairof opposing through-vias 5 b may simply be brought closer to each other.Alternatively, each through-via 5 b may be shaped as a right-angledprism such that one side thereof extends in parallel to the end of thesubstrate.

[0097] As described above, the designed frequency is defined as an upperlimit frequency value of the frequency range of a high-frequency signalto be transmitted through the signal strip 2 in the present embodiment.Alternatively, the designed frequency may be defined to be twice theupper limit frequency value of the frequency range of a high-frequencysignal to be transmitted through the signal strip 2. As a result, itbecomes possible to suppress the waveguide mode also with respect to thesecond harmonic, thus further reducing the radiation loss.

[0098] It is most preferable that the through-vias 5 b are disposed sothat a distance Wd (i.e., the lateral length of region C as shown inFIG. 1B) between the terminating end of the ground conductor layer 4 andthe portion of the through-via 5 b that lies closest to an end of thesubstrate is less than ¼ of the effective wavelength corresponding tothe designed frequency within the dielectric substrate 1, in order toachieve a good reduction of radiation loss. If the through-via 5 b isformed at a position which is ¼ of the effective wavelength or more awayfrom the terminating end of the ground conductor layer 4, a higher-moderesonance with a frequency corresponding to ¼ of the effectivewavelength will occur in the portion between the through-via 5 b and theterminating end of the ground conductor layer, resulting in an increasedradiation loss in a frequency range near the resonance frequency.

[0099] Although the present embodiment illustrates an example where thethrough-via has a columnar shape, the through-via may be shaped as arectangular column, a triangular column, or a hexagonal column, forexample.

[0100] Although the dielectric substrate is illustrated as having a“lower face” on which the signal strips and the ground strips are formedand an “upper face” on which the ground conductor layer is formed, thelower and upper faces may be reversed.

[0101] The dielectric substrate may advantageously be a resin substratehaving a low dielectric constant. Generally speaking, a resin substrate,a ceramic substrate, or the like is usable as a dielectric substrate.Note that the effective wavelength within the dielectric substrate willdecrease as the dielectric constant of the material of the dielectricsubstrate increases. Therefore, employing a dielectric substrate havinga high dielectric constant will require a high precision in theformation of wiring patterns, possibly causing characteristicfluctuations due to manufactural fluctuations. Therefore, by employing alow-dielectric constant dielectric substrate, e.g., having a dielectricconstant of 5.0 or less, the characteristic fluctuations can be reduced,thereby facilitating the obtainment of the desired effects. The presentinvention is advantageous over the conventional techniques not onlybecause the present invention provides a solution based on a circuitstructure which requires a less volumetric space, but also because itrequires no special processes during manufacture.

[0102] (Second Embodiment)

[0103]FIG. 3 is an enlarged view showing a wiring pattern on ahigh-frequency circuit according to a second embodiment of the presentinvention. In FIG. 3, those elements which have their counterparts inthe first embodiment are denoted by the same reference numerals as thoseused therein. As shown in FIG. 3, a width Ws2 of the signal strip 2 atits terminating end is smaller than a width Ws1 thereof at any pointother than the terminating end.

[0104] By thus reducing the width Ws2 of the signal strip 2 at itsterminating end, the capacitance between the ground conductor layer 4and the signal strip 2 can be reduced, relative to which the capacitancebetween the signal strip 2 and the ground strips 3 will appear large. Asa result, a smooth transition from a microstrip line-based transmissionmode to a coplanar waveguide-based transmission mode occurs, thusreducing reflections of the high-frequency signal to be transmitted.

[0105] The present embodiment will be particularly useful in thefollowing case, for example. As can be seen from FIG. 3,W=2×Wp+2×Wg1+Ws1. Similarly, the distance Wa (or Wc) between the pair ofopposing through-vias 5 b is defined by the width of the signal strip 2,the interspace between the signal strip 2 and either ground strip 3, andthe shortest distance from the ground strip 3 to the surface of eachthrough-via. Thus, the actual prescription of the value of the opposingdistance Wa (or Wc) must be made in compliance with the particularprocess rules adopted. For example, some process rules imposelimitations on the values of Wg1 and Wp; in this case, the desired Wa(or Wc) value may be obtained not by adjusting Wg1 or Wp, but byadjusting Ws (hence the reduced width Ws2 of the signal strip 2 at theterminating end). Thus, according to the second embodiment, ahigh-frequency circuit with a reduced radiation loss and subduedreflections can be provided even in the case where the adopted processrules impose limitations on Wg1 or Wp.

[0106] (Third Embodiment)

[0107]FIG. 4 is an enlarged view showing a wiring pattern on ahigh-frequency circuit according to a third embodiment of the presentinvention. In FIG. 4, those elements which have their counterparts inthe first embodiment are denoted by the same reference numerals as thoseused therein. As shown in FIG. 4, an interspace Wg2 between the signalstrip 2 and either ground strip 3 is smaller at the terminating end ofthe signal strip 2 than an interspace Wg1 at any point other than theterminating end.

[0108] By thus reducing the interspace Wg2 between the signal strip 2and either ground strip 3 at the terminating end of the signal strip 2,the capacitance between the signal strip 2 and the ground strips 3 canbe increased, relative to which the capacitance between the groundconductor layer 4 and the signal strip 2 will appear small. As a result,a smooth transition from a microstrip line-based transmission mode to acoplanar waveguide-based transmission mode occurs, thus reducingreflections of the high-frequency signal to be transmitted.

[0109] The present embodiment will be particularly useful in thefollowing case, for example. As can be seen from FIG. 4,W=2×Wp+2×Wg1+Ws1. Similarly, the distance Wa (or Wc) between the pair ofopposing through-vias 5 b is defined by the width of the signal strip 2,the interspace between the signal strip 2 and either ground strip 3, andthe shortest distance from the edge of either ground strip 3 to thesurface of each through-via. Thus, the actual prescription of the valueof the opposing distance Wa (or Wc) must be made in compliance with theparticular process rules adopted. For example, some process rules imposelimitations on the values of Wp and Ws1; in this case, the desired Wa(or Wc) value may be obtained not by adjusting Wp or Ws1, but byadjusting Wg (hence the reduced interspace Wg2 at the terminating end ofthe signal strip 2). Thus, according to the third embodiment, ahigh-frequency circuit with a reduced radiation loss and subduedreflections can be provided even in the case where the adopted processrules impose limitations on Wp or Ws1.

[0110] (Fourth Embodiment)

[0111]FIGS. 5A and 5B are enlarged views each showing a wiring patternof a high-frequency circuit according to a fourth embodiment of thepresent invention. In FIGS. 5A and 5B, those elements which have theircounterparts in the first embodiment are denoted by the same referencenumerals as those used therein. FIG. 5A shows a wiring pattern on alower face of the dielectric substrate 1, and FIG. 5B shows a wiringpattern on an upper face of the dielectric substrate 1.

[0112] As shown in FIG. 5A, the wiring pattern on the lower face of thedielectric substrate 1 is similar to that in the first embodiment. Onthe other hand, as shown in FIG. 5B, the wiring pattern on the upperface of the dielectric substrate 1 is different from that in the firstembodiment with respect the ground conductor layer 4 a.

[0113] As shown in FIG. 5B, portions of the ground conductor layer 4 aopposing the terminating end of the signal strip 2 are removed.Specifically, within region D extending from an imaginary lineconnecting “far ends” of the through-vias 5 b, i.e., portions that lieclosest to an end of the substrate (away from the center of thesubstrate) toward an end of the dielectric substrate 1 (shown as theleftmost end in FIG. 5B), those portions opposing the signal strip 2 areremoved. As a result, the parallel plane mode is suppressed, whereby theradiation loss can be reduced.

[0114] Alternatively, the portions of the ground conductor layer 4 to beremoved may be portions interposed between the pair of through-vias 5 band opposing the signal strip 2. In this case, the parallel plane modewill be suppressed, whereby the radiation loss can be reduced.

[0115] Although the present embodiment illustrates an example where thewiring pattern according to the first embodiment is employed, it will beappreciated that a wiring pattern according to the second or thirdembodiment may instead be employed.

[0116] (Fifth Embodiment)

[0117]FIGS. 6A, 6B, and 6C are views illustrating a high-frequencypackage according to a fifth embodiment of the present invention havingbeen surface-mounted on an external circuit substrate FIG. 6A is aschematic cross-sectional view; FIG. 6B is a view illustrating a wiringpattern of conductors formed on an upper face of a dielectric substrate1 shown in FIG. 6A; and FIG. 6C is a view illustrating a wiring patternof conductors formed on a lower face of the dielectric substrate 1 shownin FIG. 6A. In FIGS. 6A, 6B, and 6C, those elements which have theircounterparts in the first embodiment are denoted by the same referencenumerals as those used therein.

[0118] As used herein, a “high-frequency package” at least comprises: ahigh-frequency circuit constructed on a dielectric substrate; and ahigh-frequency element (which in itself is composed of an integratedcircuit) mounted on the high-frequency circuit.

[0119] In FIG. 6A, the high-frequency package comprises a high-frequencyelement 10 which comprises an integrated circuit for processing ahigh-frequency signal, a dielectric substrate 1, and a cover 9. Thehigh-frequency package is surface-mounted on an external circuitsubstrate 13, which is composed of a dielectric material. As such, theexternal circuit substrate 13 may hereinafter be referred to as a“mounting-side dielectric substrate”. As shown in FIG. 6B, on the upperface of the dielectric substrate 1, a ground conductor layer 4, twosignal strips 2 a, and a ground conductor region 4 b are formed. Asshown in FIG. 6C, on the lower face of the dielectric substrate 1, twosignal strips 2, two ground strips 3 which are disposed so as to leave apredetermined space from the signal strips 2, and a ground conductorregion 4 c are formed. The signal strips 2 a and 2, the ground conductorlayer 4, and the ground strips 3 together constitute a grounded coplanarwaveguide structure.

[0120] One end of each signal strip 2 a is connected to thehigh-frequency element 10 via a wire 11. The wire 11 may be a ribbon orthe like. The high-frequency element 10 may be mounted face down, viaconductor bumps. In other words, the high-frequency element 10 may bemounted through wireless bonding, e.g., flip chip mounting. The otherend of each signal strip 2 a is connected to one end of a correspondingsignal strip 2, via a through-via 12 for connection purposes whichpenetrates the dielectric substrate 1. Thus, a high-frequency signalwhich is output from the high-frequency element 10 or a high-frequencysignal which is input to the high-frequency element 10 is transmittedvia the wires 11, the signal strips 2 a, the through-vias 12, and thesignal strips 2, without being grounded.

[0121] On the upper face of the dielectric substrate 1, the groundconductor region 4 b is disposed directly under the high-frequencyelement 10, so as to be electrically connected to the ground conductorlayer 4. Via a plurality of through-vias 4 d penetrating the dielectricsubstrate 1, the ground conductor region 4 b is connected to the groundconductor region 4 c formed on the lower face of the dielectricsubstrate 1. The ground conductor region 4 c is electrically connectedto the ground strip 3. Thus, a high-frequency ground is provided in theground conductor region 4 d. An arbitrary number of through-vias 5 a andfour through-vias 5 b are formed between the ground conductor layer 4and the respective ground strips 3, each pair of through-vias 5 b beingprovided near an end of the dielectric substrate 1. The through-vias 5 aelectrically connect the ground strips 3 to the ground conductor layer 4to provide a better high-frequency grounding ability. Each pair ofopposing through-vias 5 b are placed apart from each other by a distancewhich effectively suppresses the waveguide mode, as described in thefirst embodiment.

[0122]FIG. 7A is a view illustrating an exemplary wiring pattern ofconductors formed on an upper face of the external circuit substrate 13.FIG. 7B is a view illustrating an exemplary wiring pattern of conductorsformed on a lower face of the external circuit substrate 13.

[0123] The external circuit substrate 13 is a substrate on which thehigh-frequency package is to be surface-mounted. As shown in FIG. 7A, onthe upper face of the external circuit substrate 13, two signal strips14, two ground strips 16, and a ground conductor region 16 b are formed.As shown in FIG. 7B, on the lower face of the external circuit substrate13, a ground conductor layer 15 is formed.

[0124] Each signal strip 14 is electrically connected to a correspondingsignal strip 2 via solder 17. Each ground strip 16 is electricallyconnected to a corresponding ground strip 3 via solder 17. Interspacesare provided between each signal strip 14 and the ground strips 16.

[0125] The ground conductor region 16 b is disposed so as to comedirectly below the high-frequency element 10. The ground conductorregion 16 b is electrically connected to the ground conductor region 4 cvia solder 17. The ground conductor region 16 b is connected to theground conductor layer 15 by means of through-vias 16 d penetrating theexternal circuit substrate 13. As a result, a high-frequency ground isprovided in the ground conductor region 16 d. An arbitrary number ofthrough-vias 5 c and four through-vias 5 d are formed between the groundconductor layer 15 and the respective ground strips 16, each pair ofthrough-vias 5 b being provided near an end of the dielectric substrate1. The through-vias 5 c electrically connect the ground strips 16 to theground conductor layer 15 to provide a better high-frequency groundingability. Each pair of opposing through-vias 5 d are disposed with adistance which effectively suppresses the waveguide mode, as describedin the first embodiment.

[0126] Due to the aforementioned strip line structure, the externalcircuit substrate 13 functions as a grounded coplanar waveguide in whicha high-frequency signal which is output from the high-frequency element10 or a high-frequency signal which is input to the high-frequencyelement 10 can be transmitted without being grounded.

[0127] Thus, according to the fifth embodiment, on both the dielectricsubstrate and the external circuit substrate, a pair of opposingthrough-vias provided near the terminating end of a signal strip aredisposed with a distance which effectively suppresses the waveguidemode. As a result, the waveguide mode can be suppressed at theterminating end of the signal strip, whereby the radiation loss can bereduced, and a mounting structure which can suppress the transmissionloss can be provided.

[0128] Although the fifth embodiment illustrates an example where thesignal strip has a uniform width, a signal strip which has a narrowerwidth at its terminating end, such as that described in the secondembodiment, may instead be used on the dielectric substrate and/or onthe external circuit substrate.

[0129] Although the fifth embodiment illustrates an example where theground strips have a uniform width, ground strips which have a thickerwidth at the terminating end of the signal strip, such as thosedescribed in the third embodiment, maybe used instead.

[0130] In the fifth embodiment, too, a portion of the ground conductorlayer on the dielectric substrate and/or on the external circuitsubstrate may be removed as in the fourth embodiment.

[0131] The fifth embodiment illustrates an example where a groundedcoplanar waveguide is constructed over the entire lower face of thedielectric substrate 1 or over the entire upper face of the externalcircuit substrate 13. However, the strip line structure is not limitedto the above, so long as the principle of the present invention isapplied. For example, a part of the strip line structure may by amicrostrip line.

[0132] The ground conductor layer 15 may be formed inside the externalcircuit substrate 13. By omitting the ground strips 16, the externalcircuit substrate 13 may be allowed to function as a microstrip line.

[0133] In the above-described embodiments, it is preferable that thedesigned frequency is set equal to or greater than the upper limitfrequency value of a range which is used in a given wirelesscommunications system. By prescribing such a designed frequency, themounted high-frequency element can function without unfavorablyaffecting its gain, noise, and frequency conversion characteristics.

[0134] In the above-described embodiments, it is more preferable thatthe designed frequency is set equal to or greater than a second harmonicof the upper limit frequency value of a range which is used in a givenwireless communications system. By prescribing such a designedfrequency, the mounted high-frequency element can function withoutunfavorably affecting its distortion characteristics.

[0135] In order to ascertain actual effects of the present invention,the inventor has measured the transmission characteristics of varioushigh-frequency circuits according to Examples 1 to 10 described below.FIG. 8 is a view showing a generic wiring pattern on a wiring substrateon which a high-frequency element was mounted for evaluations. It willbe appreciated that, once a high-frequency element is mounted on awiring substrate for evaluations, the structure as shown in FIG. 6A(with or without the external circuit substrate 13) is obtained.

[0136] As shown in FIG. 8, each dielectric substrate 1 used forevaluations had the following dimensions: a distance W between each pairof opposing through-vias 5 a; a distance Wa between each pair ofopposing through-vias 5 b; a distance Wc between portions of thethrough-vias 5 b that lie closest to an end (shown as the leftmost end)of the dielectric substrate 1; an interspace Wg1 between a signal strip2 and either ground strip 3, taken at a central portion of the signalstrip 2; an interspace Wg2 between the signal strip 2 and either groundstrip 3, taken at a terminating end of the signal strip 2; a width Ws1of the signal strip 2 at its central portion; a width Ws2 of the signalstrip 2 at its terminating end; and a shortest distance Wp from the edgeof either ground strip 3 to the surface of each through-via 5 b, takenat the terminating end of the signal strip 2. Although FIG. 8illustrates a generic set of dimensions such that Wg1 is not equal toWg2 and that Ws1 is not equal to Ws2, it will be understood that Wg1 maybe equal to Wg2 and/or Ws1 may be equal to Ws2 in some of the Examplesand Comparative Examples below, as clarified in the descriptionaccompanying each such Example or Comparative Example

[0137] First, conditions which were common to all of Examples 1 to 10are described. In all of Examples 1 to 10, a liquid crystal polymersubstrate having a dielectric constant of 3 and a thickness of 125micrometers was used as the dielectric substrate 1. On the entire upperface of the dielectric substrate 1, a ground conductor layer 4 wasformed except for regions within 100 micrometers from the rightmost andleftmost end. Note that the ground conductor layer 4 is disposed on theother side of the dielectric substrate 1 which cannot be drawn in FIG.8, and is therefore labeled as “4” via a dotted line.

[0138] On the lower face of the dielectric substrate 1, the signal strip2 (having the width Ws1 at its central portion) and two ground strips 3(each having a width of 600 micrometers) were formed, such that the twoground strips 3 interposed the signal strip 2 with the distance of Wg1left between either ground strip 3 and the signal strip 2. Thedielectric substrate 1 had a length of 2000 micrometers along adirection of signal transmission.

[0139] The wiring rules dictate that no conductor should be formed atthe very ends of the dielectric substrate 1, which is the reason why thedielectric substrate 1 had a region of 100 micrometers from therightmost and leftmost end where no conductor is formed. The conductorpattern was composed of copper with a thickness of 40 micrometers. Thethrough-vias 5 a and 5 b connecting the ground strips 3 to the groundconductor layer 4 were formed by drilling apertures with a radius of 100micrometers through the dielectric substrate 1, and thereafter platinginner walls of the apertures to an average thickness of 20 micrometersin order to confer conductivity thereto. A void was partially left ineach of the through-vias 5 a and 5 b.

[0140] Due to process rule limitations, the minimum value of theshortest distance Wp from the edge of either ground strip 3 to thesurface of each through-via 5 b was set to 200 micrometers.

[0141] As the external circuit substrate, a Teflon® substrate having adielectric constant of 2.5 and a thickness of 200 micrometers was used.The process rules used for producing the external circuit substrate werethe same as the process rules for the dielectric substrate 1.

[0142] Table 1 shows S21 and MAG measurements (at 87 GHz) according toComparative Examples 1 and 2 and Examples 1 and 2. As used herein, “S21”is one of the S parameters concerning transmission characteristics thatindicates passing intensity. MAG (Maximum Available Gain) is an index ofloss, from which the influences of passage loss-based degradations dueto impedance mismatching at the input or output terminal are eliminated.As such, MAG can be used as a more quantitative index of radiation loss.Hereinafter, Comparative Examples 1 and 2 and Examples 1 and 2 will bedescribed with reference to Table 1. TABLE 1 Positioning of through-viapairs on dielectric substrate 1 S21 MAG Ws2 Wg2 Wa Wc W (87 GHz)/ (87GHz)/ (μm) (μm) (μm) (μm) (μm) dB dB Comparative 200 200 1000 1200 1000−4.79 −2.21 Example 1 Comparative 200 150 1000 1200 1000 −5.36 −2.22Example 2 Example 1 200 150 950 1150 1000 −4.21 −2.01 Example 2 200 150900 1100 1000 −3.41 −1.81

[0143] As common conditions among Comparative Examples 1 and 2 andExamples 1 and 2, Ws1 was prescribed to be 200 micrometers and Wg1 wasprescribed to be 200 micrometers, thus constructing a grounded coplanarwaveguide having a characteristic impedance 15 of about 50 Ω. Theshortest distance W between opposing through-vias 5 a was prescribed tobe 1000 micrometers. For a 85 GHz signal, a half of the effectivewavelength within the dielectric substrate 1 equals 1020 micrometers.Therefore, the cutoff frequency for the waveguide mode is about 85 GHz,i.e., the waveguide mode begins to occur around a frequency of 85 GHz.Thus, it can be seen that Examples 1 and 2 and Comparative Examples 1and 2 were designed to have a designed frequency of 85 GHz.

[0144] In Comparative Example 1, Ws2 was 200 micrometers; Wg2 was 200micrometers; and the distance Wa between opposing through-vias 5 b was1000 micrometers. Since the radius of the through-vias 5 b was 100micrometers, Wc was 1200 micrometers. As a result, S21 at 87 GHz wasmeasured to be −4.79 dB, and MAG at 87 GHz was measured to be −2.21 dB.

[0145] In Comparative Example 2, Ws2 was 200 micrometers; Wg2 was 150micrometers; and the distance Wa between opposing through-vias 5 b was1000 micrometers. As a result, S21 at 87 GHz was measured to be −5.36dB, and MAG at 87 GHz was measured to be −2.22 dB.

[0146] In Example 1, Ws2 was 200 micrometers; Wg2 was 150 micrometers;and Wa was 950 micrometers. Thus, Wc was 1150 micrometers. Since Wa was950 micrometers, the cutoff frequency for the waveguide mode near theinterspace between each pair of opposing through-vias 5 b was about 91GHz. As a result, S21 at 87 GHz was measured to be −4.21 dB, and MAG at87 GHz was measured to be −2.01 dB.

[0147] In Example 2, Ws2 was 200 micrometers; Wg2 was 150 micrometers;and Wa was 900 micrometers. Thus, Wc was 1100 micrometers. Since Wa was900 micrometers, the cutoff frequency for the waveguide mode near theinterspace between each pair of opposing through-vias 5 b was about 96GHz. As a result, S21 for 87 GHz was measured to be −3.41 dB, and MAGfor 87 GHz was measured to be −1.81 dB.

[0148] In Table 1, one can see from comparison against ComparativeExample 1 that the waveguide mode was suppressed and the passage losswas reduced in Examples 1 and 2, by ensuring that the distance Wabetween each pair of opposing through-vias 5 b located closest to an endof the substrate was shorter than the distance W between other pairs ofopposing through-vias 5 a, such that Wa was less than ½ of the effectivewavelength corresponding to the designed frequency of 85 GHz.

[0149] It was anticipated that passage loss degradation would also occurif there was mismatching between the input or output terminal and a 50 Ωmeasurement system. Therefore, as a more quantitative index of radiationloss, Table 1 also shows MAG measurements, from which the influences ofpassage loss-based degradations due to impedance mismatching at theinput or output terminal are eliminated. From comparison between the MAGmeasurements, one could also see that the waveguide mode was suppressedand the radiation loss was reduced by the use of the decreased distancebetween each pair of opposing through-vias 5 b located closest to an endof the substrate according to the present invention. Note that, bycomparing Comparative Examples 1 and 2, it can be seen that the use of areduced Wg2 value at an end of the substrate would lead to a mismatchingand hence an increased passage loss. Nonetheless, the passage loss wasreduced in Examples 1 and 2, despite the use of the same Ws2 and Wg2values as in Comparative Example 2. Therefore, it is evident thatreduction in passage loss in Examples 1 and 2 as compared to ComparativeExample 1 was not due to an alleviation of mismatching, but due to areduction in radiation loss.

[0150] Table 2 shows S21 and MAG measurements (at 87 GHz) according toComparative Example 3 and Examples 3 and 4. Hereinafter, ComparativeExample 3 and Examples 3 and 4 will be described with reference to Table2. TABLE 2 Positioning of through-via pairs on dielectric substrate 1S21 MAG Ws2 Wg2 Wa Wc W (87 GHz)/ (87 GHz)/ (μm) (μm) (μm) (μm) (μm) dBdB Comparative 150 150 1000 1200 1000 −6.21 −1.88 Example 3 Example 3150 150 900 1100 1000 −4.02 −1.59 Example 4 150 150 850 1050 1000 −3.39−1.47

[0151] As common conditions among Comparative Example 3 and Examples 3and 4, Ws1 was prescribed to be 200 micrometers; Wg1 was prescribed tobe 200 micrometers; Ws2 was prescribed to be 150 micrometers; Wg2 wasprescribed to be 150 micrometers, and W was prescribed to be 1000micrometers.

[0152] In Comparative Example 3, Wa was 1000 micrometers. As a result,S21 at 87 GHz was measured to be −6.21 dB, and MAG at 87 GHz wasmeasured to be −1.88 dB.

[0153] In Example 3, Wa was 900 micrometers. Since the radius of thethrough-vias 5 b was 100 micrometers, the distance Wc between portionsof the opposing through-vias 5 b that lie closest to an end of thesubstrate 1 was 1100 micrometers. Under this condition, by using Wainstead of W, the cutoff frequency for the waveguide mode near theinterspace between each pair of opposing through-vias 5 b would becalculated to be about 91 GHz. Similarly, by using Wc instead of W, thecutoff frequency for the waveguide mode near the interspace between eachpair of opposing through-vias 5 b would be calculated to be about 78GHz. As a result, S21 at 87 GHz was measured to be −4.02 dB, and MAG at87 GHz was measured to be −1.59 dB.

[0154] In Example 4, Wa was 850 micrometers. Since the radius of thethrough-vias 5 b was 100 micrometers, the distance Wc between portionsof the opposing through-vias 5 b that lie closest to an end of thesubstrate 1 was 1050 micrometers. Under this condition, by using Wainstead of W, the cutoff frequency for the waveguide mode near theinterspace between each pair of opposing through-vias 5 b would becalculated to be about 96 GHz. Similarly, by using Wc instead of W, thecutoff frequency for the waveguide mode near the interspace between eachpair of opposing through-vias 5 b would be calculated to be about 82.5GHz. As a result, S21 at 87 GHz was measured to be −3.39 dB, and MAG at87 GHz was measured to be −1.47 dB.

[0155] In Table 2, one can see from comparison against ComparativeExample 3 that the waveguide mode was suppressed and the passage losswas reduced in Examples 3 and 4, by prescribing a decreased distancebetween each pair of opposing through-vias 5 b located closest to an endof the substrate according to the present invention, i.e., less than ½of the effective wavelength corresponding to the designed frequency of85 GHz. From comparison between the MAG measurements, one could also seethat the waveguide mode was suppressed and the radiation loss wasreduced by the use of the decreased distance between each pair ofopposing through-vias 5 b located closest to an end of the substrateaccording to the present invention.

[0156] Table 3 shows S21 and MAG measurements (at 87 GHz) according toComparative Examples 4 and 5 and Examples 5 to 7. Hereinafter,Comparative Examples 4 and 5 and Examples 5 to 7 will be described withreference to Table 3. TABLE 3 Positioning of through-via pairs S21 ondielectric substrate 1 (87 MAG Ws2 Wg2 Wa Wc W GHz)/ (87 GHz)/ (μm) (μm)(μm) (μm) (μm) dB dB Comparative 150 100 1000 1200 1000 −6.43 −1.84Example 4 Example 5 150 100 900 1100 1000 −3.78 −1.58 Example 6 150 100800 1000 1000 −2.47 −1.37 Example 7 150 100 750 950 1000 −2.39 −1.36Comparative 150 100 1000 1200  1000, −5.8  −1.96 Example 5 800

[0157] As common conditions among Comparative Example 4 and Examples 5to 7, Ws1 was prescribed to be 200 micrometers; Wg1 was prescribed to be200 micrometers; Ws2 was prescribed to be 150 micrometers; Wg2 wasprescribed to be 100 micrometers; and W was prescribed to be 1000micrometers.

[0158] In Comparative Example 4, Wa was 1000 micrometers. As a result,S21 at 87 GHz was measured to be −6.43 dB, and MAG at 87 GHz wasmeasured to be −1.84 dB.

[0159] In Example 5, Wa was 900 micrometers. Thus, Wc was 1100micrometers. Under this condition, by using Wa instead of W, the cutofffrequency for the waveguide mode near the interspace between each pairof opposing through-vias 5 b would be calculated to be about 91 GHz.Similarly, by using Wc instead of W, the cutoff frequency for thewaveguide mode near the interspace between each pair of opposingthrough-vias 5 b would be calculated to be about 78 GHz. As a result,S21 at 87 GHz was measured to be −3.78 dB, and MAG at 87 GHz wasmeasured to be −1.58 dB.

[0160] In Example 6, Wa was 800 micrometers. Thus, Wc was 1000micrometers. Under this condition, by using Wa instead of W, the cutofffrequency for the waveguide mode near the interspace between each pairof opposing through-vias 5 b would be calculated to be about 101 GHz.Similarly, by using Wc instead of W, the cutoff frequency for thewaveguide mode near the interspace between each pair of opposingthrough-vias 5 b would be calculated to be about 87 GHz. As a result,S21 at 87 GHz was measured to be −2.47 dB, and MAG at 87 GHz wasmeasured to be −1.37 dB.

[0161] In Example 7, Wa was 750 micrometers. Thus, Wc was 950micrometers. Under this condition, by using Wa instead of W, the cutofffrequency for the waveguide mode near the interspace between each pairof opposing through-vias 5 b would be calculated to be about 106 GHz.Similarly, by using Wc instead of W, the cutoff frequency for thewaveguide mode near the interspace between each pair of opposingthrough-vias 5 b would be calculated to be about 91 GHz. As a result,S21 at 87 GHz was measured to be −2.39 dB, and MAG at 87 GHz wasmeasured to be −1.36 dB.

[0162] In Comparative Example 5, Wa was for the most part 1000micrometers, except that the opposing distance W between each of twopairs of opposing through-vias 5 a (such that each pair was located thesecond closest to an end of the substrate) was 800 micrometers. As aresult, S21 at 87 GHz was measured to be −5.8 dB, and MAG at 87 GHz wasmeasured to be −1.96 dB.

[0163] In Table 3, one can see from comparison against ComparativeExample 4 that the waveguide mode was suppressed and the passage losswas reduced in Examples 5 to 7, by prescribing a decreased distancebetween each pair of opposing through-vias 5 b located closest to an endof the substrate according to the present invention, i.e., less than ½of the effective wavelength corresponding to the designed frequency of85 GHz. From comparison between the MAG measurements, one could also seethat the waveguide mode was suppressed and the radiation loss wasreduced by the use of the decreased distance between each pair ofopposing through-vias 5 b located closest to an end of the substrateaccording to the present invention.

[0164] Moreover, in the region at either end of the substrate, thecondition for preventing the 87 GHz signal to be transmitted from beingconverted to the waveguide mode is that the distance Wc between portionsof the opposing through-vias 5 b that lie closest to an end of thesubstrate 1 be less than 1000 micrometers, i.e., ½ of the effectivewavelength corresponding to the designed frequency of 87 GHz within thedielectric substrate 1. The validity of the principle of the presentinvention is clear from the fact that the passage loss and MAGmeasurements for Example 6, which sufficiently satisfies this condition,and Example 7, which employs an even shorter value of Wc, each convergeto toward a similar value.

[0165] A comparison between Comparative Examples 4 and 5 would indicatethe following. In Comparative Examples 4 and 5, Wg2 was reduced to 100micrometers in order to obtain a minimum Wp value of 200 micrometers inthe neighborhood of the through-vias 5 a and 5 b. As shown in Table 3,there are differences in matching between Comparative Examples 4 and 5,so that the characteristics of Comparative Examples 4 and 5 are notentirely identical. However, the characteristics of Comparative Example5 are closer to those of Comparative Example 4 than those of Example 6,in terms of both passage loss and MAG. In other words, the improvedcharacteristics of Example 6 as compared to Comparative Example 4 arenot obtained in Comparative Example 5, which employ a decreased distancebetween opposing through-vias 5 a. Thus, it is clear that what is mosteffective according to the present invention is to reduce the distanceWa between a pair of opposing through-vias 5 b in the region near eitherend of the substrate, rather than reducing the distance W between anypair of opposing through-vias 5 a in regions other than the ends of thesubstrate.

[0166] Table 4 shows S21 and MAG measurements (at 87 GHz) according toComparative Examples 4 and 6 and Examples 6 and 8. Hereinafter,Comparative Examples 4 and 6 and Examples 6 and 8 will be described withreference to Table 4. TABLE 4 partial Positioning of through-via removalpairs on dielectric of S21 MAG substrate 1 ground (87 (87 Ws2 Wg2 Wa WcW conduc- GHz)/ GHz)/ (μm) (μm) (μm) (μm) (μm) tor layer dB dB Com- 150100 1000 1200 1000 NO −6.43 −1.84 parative Exam- ple 4 Com- 150 100 10001200 1000 YES −4.93 −0.99 parative Exam- ple 6 Exam- 150 100 800 10001000 NO −2.47 −1.37 ple 6 Exam- 150 100 800 1000 1000 YES −1.55 −0.65ple 8

[0167] The conditions and measurement results of Comparative Example 4and Example 6 were the same as those shown in Table 3. ComparativeExample 6 was constructed similarly to Comparative Example 4 except thatthe ground conductor layer 4 formed on the upper face of the dielectricsubstrate 1 was partially removed. As shown in FIG. 5B, the portionswhich were removed from the ground conductor layer 4 were, within aregion located near either end of the substrate and away from the regionwhere the through-vias 5 b oppose each other, portions opposing aterminating end of the signal strip 2. Example 8 was constructedsimilarly to Example 6 except that the ground conductor layer 4 waspartially removed in the same manner as in Comparative Example 6.

[0168] In Table 4, one can see from comparing the characteristics ofExamples 6 and 8 that a partial removal of the ground conductor layer 4improved the passage loss and MAG characteristics.

[0169] Comparative Examples 4 and 6 indicate that some improvement inthe passage loss and MAG characteristics could be obtained by merelyremoving portions of the ground conductor layer 4. Nevertheless, itshould be clear from Example 8 and Comparative Example 6 that a moredrastic improvement in the characteristics can be obtained by reducingthe distance Wa between opposing through-vias 5 b in addition toremoving portions of the ground conductor layer 4.

[0170] Table 5 shows S21 and MAG measurements (at 87 GHz) according toExamples 7, 9, and 10. Hereinafter, Examples 7, 9, and 10 will bedescribed with reference to Table 5. TABLE 5 Positioning of through-viapairs Positioning of (closest through-via pairs on to end) on dielectricsubstrate 1 external S21 MAG Ws2 Wg2 Wa Wc W circuit (87 GHz)/ (87 GHz)/(μm) (μm) (μm) (μm) (μm) substrate dB dB Example 7 150 100 750 950 1000same as −2.39 −1.36 conven-tional Example 9 150 100 750 950 1000inventive −1.8  −1.22 (900 μm apart) Example 10 150 100 750 950 1000inventive −1.75 −1.19 (800 μm apart)

[0171] In Examples 9 and 10, the wiring pattern on the dielectricsubstrate 1 was similar to that in Example 7; however, the through-viapattern described in the fifth embodiment of the present invention wasadopted for the external circuit substrate 13. The following descriptionwill therefore refer to FIGS. 7A and 7B.

[0172] In Example 7, among the through-vias 5 c and 5 d connecting theground conductor layer 15 formed on the lower face of the externalcircuit substrate 13 to the ground strips 16 disposed on both sides ofthe signal strip 14 formed on the upper face of the external circuitsubstrate 13, each pair of opposing through-vias 5 d near theterminating end of each signal strip 14 were disposed apart by theconventional distance of 1000 micrometers. Under this condition, byusing the distance between each pair of opposing through-vias 5 blocated closest to the connecting portions, the cutoff frequency for thewaveguide mode in signal transmission within the external circuitsubstrate would be calculated to be about 94 GHz. Meanwhile, since theradius of the through-vias 5 d was 100 micrometers, the opposingdistance between portions of the through-vias 5 d that lie closest tothe connection with an external element was 1200 micrometers. Under thiscondition, the cutoff frequency for the waveguide mode near theconnection would be calculated to be about 78 GHz. As a result, S21 at87 GHz was −2.39 dB, and MAG at 87 GHz was −1.36 dB.

[0173] In Example 9, the distance between opposing through-vias 5 d was900 micrometers. Under this condition, by using the distance betweeneach pair of opposing through-vias 5 b located closest to the connectingportions, the cutoff frequency for the waveguide mode in signaltransmission within the external circuit substrate would be calculatedto be about 104 GHz. Meanwhile, since the radius of the through-vias 5 dwas 100 micrometers, the opposing distance between portions of thethrough-vias 5 d that lie closest to the connection with an externalelement was 1100 micrometers. Under this condition, the cutoff frequencyfor the waveguide mode near the connection would be calculated to beabout 85 GHz. As a result, S21 at 87 GHz was −1.8 dB, and MAG at 87 GHzwas −1.22 dB.

[0174] In Example 10, the distance between opposing through-vias 5 d was800 micrometers. Under this condition, by using the distance betweeneach pair of opposing through-vias 5 b located closest to the connectingportions, the cutoff frequency for the waveguide mode in signaltransmission within the external circuit substrate would be calculatedto be about 118 GHz. Meanwhile, since the radius of the through-vias 5 dwas 100 micrometers, the opposing distance between portions of thethrough-vias 5 d that lie closest to the connection with an externalelement was 900 micrometers. Under this condition, the cutoff frequencyfor the waveguide mode near the connection would be calculated to beabout 104 GHz. As a result, S21 at 87 GHz was −1.75 dB, and MAG at 87GHz was −1.19 dB.

[0175] In Table 5, one can see from comparison against Example 7 thatboth the passage loss and the MAG characteristics were more improved inExamples 9 and 10. Thus, it is clear that on the external circuitsubstrate 13, too, the advantageous effects of suppressing the waveguidemode and reducing the passage loss can be obtained by reducing thedistance between opposing through-vias 5 d located closest to an end ofthe substrate so as to be less than ½ of the effective wavelengthcorresponding to the designed frequency of 94 GHz.

[0176] The high-frequency circuit according to the present invention canreduce the loss associated with the transmission of high-frequencysignals, and is therefore useful in fields such as communications.

[0177] While the invention has been described in detail, the foregoingdescription is in all aspects illustrative and not restrictive. It isunderstood that numerous other modifications and variations can bedevised without departing from the scope of the invention.

What is claimed is:
 1. A high-frequency circuit formed on a surface of adielectric substrate, comprising: a signal strip formed on a first faceof the dielectric substrate for transmitting a signal therethrough; apair of ground strips formed on the first face astride the signal strip,with an interspace on each side of the signal strip; a ground conductorlayer formed on a second face of the dielectric substrate, the secondface being opposite to the first face; and a plurality of through-viasformed in the dielectric substrate astride the signal strip forelectrically connecting the pair of ground strips to the groundconductor layer, wherein, among the plurality of through-vias, first andsecond through-vias which are a pair of opposing through-vias locatedclosest to a terminating end of the signal strip are disposed apart fromeach other by a distance smaller than a distance between any other pairof opposing through-vias.
 2. The high-frequency circuit according toclaim 1, wherein, the distance between the first and second through-viasis less than ½ of an effective wavelength corresponding to a designedfrequency, and the distance between any other pair of opposingthrough-vias is equal to or less than ½ of the effective wavelengthcorresponding to the designed frequency.
 3. The high-frequency circuitaccording to claim 2, wherein, the terminating end of the signal stripis near an end of the dielectric substrate, and the first and secondthrough-vias are disposed so that a distance between portions thereofthat lie closest to the end of the dielectric substrate is less than ½of the effective wavelength corresponding to the designed frequency. 4.The high-frequency circuit according to claim 1, wherein the first andsecond through-vias are each disposed so as to be away from an end ofthe ground conductor layer by a distance which is less than ¼ of theeffective wavelength corresponding to the designed frequency.
 5. Thehigh-frequency circuit according to claim 1, wherein the signal strip isnarrower at the terminating end than at any other portion.
 6. Thehigh-frequency circuit according to claim 1, wherein an interspacebetween the signal strip and each ground strip is narrower at theterminating end of the signal strip than at any other portion.
 7. Thehigh-frequency circuit according to claim 1, wherein the dielectricsubstrate is a resin substrate having a low dielectric constant.
 8. Thehigh-frequency circuit according to claim 1, wherein portions of theground conductor layer interposed between the first and the secondthrough-via and opposing the signal strip are eliminated.
 9. Thehigh-frequency circuit according to claim 1, wherein, within a regionextending nearer to an end of the substrate than to a region interposedbetween the first and second through-vias, portions of the groundconductor layer opposing the signal strip are eliminated.
 10. Thehigh-frequency circuit according to claim 1, wherein portions of theground conductor layer opposing the signal strip are removed.
 11. Ahigh-frequency package into which an integrated circuit is packaged,comprising: a high-frequency element composed of the integrated circuitfor processing a high-frequency signal; and a dielectric substrate onwhich the high-frequency element is mounted, wherein the dielectricsubstrate includes: a signal strip formed on a first face of thedielectric substrate for transmitting a signal therethrough; a pair ofground strips formed on the first face astride the signal strip, with aninterspace on each side of the signal strip; a ground conductor layerformed on a second face of the dielectric substrate, the second facebeing opposite to the first face; and a plurality of through-vias formedin the dielectric substrate astride the signal strip for electricallyconnecting the pair of ground strips to the ground conductor layer,wherein, among the plurality of through-vias, first and secondthrough-vias which are a pair of opposing through-vias located closestto a terminating end of the signal strip are disposed apart from eachother by a distance smaller than a distance between any other pair ofopposing through-vias.
 12. The high-frequency package according to claim11, wherein, the distance between the first and second through-vias isless than ½ of an effective wavelength corresponding to a designedfrequency, and the distance between any other pair of opposingthrough-vias is equal to or less than ½ of the effective wavelengthcorresponding to the designed frequency.
 13. The high-frequency packageaccording to claim 12, wherein, the terminating end of the signal stripis near an end of the dielectric substrate, and the first and secondthrough-vias are disposed so that a distance between portions thereofthat lie closest to the end of the dielectric substrate is less than ½of the effective wavelength corresponding to the designed frequency. 14.The high-frequency package according to claim 11, further comprising amounting-side dielectric substrate on which the dielectric substrate ismounted, wherein the mounting-side dielectric substrate includes: amounting-side signal strip formed on a first mounting face of themounting-side dielectric substrate so as to be connected to the signalstrip for transmitting the signal therethrough; a pair of mounting-sideground strips formed on the first mounting face astride themounting-side signal strip, with an interspace on each side of themounting-side signal strip; a mounting-side ground conductor layerformed on a second mounting face of the mounting-side dielectricsubstrate, the second mounting face being opposite to the first mountingface; and a plurality of mounting-side through-vias formed in themounting-side dielectric substrate astride the mounting-side signalstrip for electrically connecting the pair of mounting-side groundstrips to the mounting-side ground conductor layer, wherein, among theplurality of mounting-side through-vias, first and second mounting-sidethrough-vias which are a pair of opposing mounting-side through-viaslocated closest to a terminating end of the mounting-side signal stripare disposed apart from each other by a distance smaller than a distancebetween any other pair of opposing mounting-side through-vias.
 15. Thehigh-frequency package according to claim 14, wherein, the distancebetween the first and second mounting-side through-vias is less than ½of an effective wavelength corresponding to a designed frequency, andthe distance between any other pair of opposing mounting-sidethrough-vias is equal to or less than ½ of the effective wavelengthcorresponding to the designed frequency.
 16. The high-frequency packageaccording to claim 15, wherein, the terminating end of the mounting-sidesignal strip is near an end of the mounting-side dielectric substrate,and the first and second mounting-side through-vias are disposed so thata distance between portions thereof that lie closest to the end of themounting-side dielectric substrate is less than ½ of the effectivewavelength corresponding to the designed frequency.
 17. Thehigh-frequency package according to claim 11, further comprising a coverfor protecting the high-frequency element.
 18. A high-frequency circuitformed on a surface of a dielectric substrate on which a high-frequencypackage is to be surface-mounted, the high-frequency package having acoplanar waveguide formed on a lower face thereof, comprising: a signalstrip formed on a first face of the dielectric substrate so as to beconnected to the high-frequency package for transmitting a signaltherethrough; a pair of ground strips formed on the first face astridethe signal strip, with an interspace on each side of the signal strip; aground conductor layer formed on a second face of the dielectricsubstrate, the second face being opposite to the first face; and aplurality of through-vias formed in the dielectric substrate astride thesignal strip for electrically connecting the pair of ground strips tothe ground conductor layer, wherein, among the plurality ofthrough-vias, first and second through-vias which are a pair of opposingthrough-vias located closest to a terminating end of the signal stripare disposed apart from each other by a distance smaller than a distancebetween any other pair of opposing through-vias.
 19. The high-frequencycircuit according to claim 18, wherein, the distance between the firstand second through-vias is less than ½ of an effective wavelengthcorresponding to a designed frequency, and the distance between anyother pair of opposing through-vias is equal to or less than ½ of theeffective wavelength corresponding to the designed frequency.
 20. Thehigh-frequency circuit according to claim 19, wherein, the terminatingend of the signal strip is near an end of the dielectric substrate, andthe first and second through-vias are disposed so that a distancebetween portions thereof that lie closest to the end of the dielectricsubstrate is less than ½ of the effective wavelength corresponding tothe designed frequency.